As anyone familiar with the literature knows, mixed-race marriages and romantic relationships suffer many more problems than single-race marriages and romantic relationships. For those familiar with human biodiversity (HBD) and sociobiology, this should come as no surprise. In evolutionary terms, one could argue that mixed-race marriages are maladaptive in that they reduce a person's overall genetic fitness - i.e. passing on copies of one's own genes. In a multiracial marriage or relationship, one is showing altruism toward a partner who shares fewer genes than a co-ethnic would share. A parent will also share fewer genes with a multiracial child than with a same-race child.

It's natural for someone to prefer a partner of the same race, as this increases a person's Darwinian fitness. J. Philippe Rushton has noted:

"[P]eople maximize their inclusive ﬁtness by marrying others similar to themselves...."

"Studies of human marriages and friendships show that people choose each other on the basis of similarity, assorting on the most genetically influenced of a set of homogenous attributes.... Darwin's theory of evolution tells us that the ultimate reason for behavior, like morphology, is to enhance inclusive fitness."

Yet, while the vast majority of people are endogamous and marry within their own race, what about those who do not? And what about those who have mixed-race children? Rushton has argued that the lower frequency of shared genes in racially mixed families might result in: less intense bonding, greater conflict, and fewer children. (There are also other problems, such as mixed-race children being unable to find organ or bone-marrow transplants. More on that later.)

Part of the answer as to why this is so lies in genetic distances, as put forward by Luigi Luca Cavalli-Sforza in Genes, Peoples, and Languages. Steve Sailer writes:

"Cavalli-Sforza's team compiled extraordinary tables depicting the "genetic distances" separating 2,000 different racial groups from each other. For example, assume the genetic distance between the English and the Danes is equal to 1.0. Then, Cavalli-Sforza has found, the separation between the English and the Italians would be about 2.5 times as large as the English-Danish difference. On this scale, the Iranians would be 9 times more distant genetically from the English than the Danish, and the Japanese 59 times greater. Finally, the gap between the English and the Bantus (the main group of sub-Saharan blacks) is 109 times as large as the distance between the English and the Danish."

Using the genetic distances outlined above, let's look at two hypothetical multiracial marriages.

An English Man and a Japanese Woman: As genetic distance figures above note, an English man would be around 59 times more closely related to a Dane than to his Japanese wife.

An English Female and a Black (Bantu) Father: Using the genetic distance figures above, the distance even widens with a white-black relationship. The English woman would be around 109 times more closely related to a Dane than to her black husband, and he would overwhelmingly be more closely related to his black co-ethnics than to his wife.

What of the mixed-race children? Parents in mixed-race relationships are not only genetically dissimilar to each other but they also have a much greater genetic distance from potential mixed-race children than from same-race children. Regarding the individual's genetic investment in the second example above, Frank Salter (On Genetic Interests, pg. 261) writes:

"For a person of English ethnicity, choosing an English spouse over a Dane gains less than one percent fitness. But choosing an English spouse over a Bantu, one yields a fitness gain of 92 percent.... The same applies in reverse order, so that a Bantu who chooses another Bantu instead of someone of English ethnicity has 92% more of his or her genes in offspring as a result. It is almost the equivalent to having twice the number of children with an English spouse. Thus assortative mating by ethnicity can have large fitness benefits, the largest derived from choosing mates within geographic races."

In other words and general terms, a white mother will be almost as twice as closely related to a child with a white father versus a child with a black father. Because same-race parents share more genes, each parent is likely to see more of his or her genes in the offspring even if they are not passed on directly. For example, if the father has gene X and doesn't pass it on directly to his son, there's a good chance his same-race spouse will have gene X and pass it on, so the son will indirectly possess the father's gene X.

Noting phenotype in mixed-race children, each parent would more closely resemble co-ethnics than their own child, especially the white mother, since whites tend to have recessive traits for appearance. (A person only 1/16 black will often still have visible and prominent black features.)

And appearance does matter. The fact that mixed-race children do not resemble the parents, esp. the fairer parent, seems to be an issue of concern, although not widely discussed. In a candid letter about having a multiracial baby with a man from India, an English mother notes:

""She's getting very dark, isn't she?" This is what one of my friends recently said about my much adored - 12-week-old daughter. She didn't mean to be rude. But it was a comment that struck me with the force of a jab to the stomach. Immediately, I was overwhelmed by a confusion of emotions. I felt protective, insulted, worried, ashamed, guilty, all at once. The reason? My lovely, wriggly, smiley baby is mixed race..... The truth is, whatever the label, the fact there is a label proves that my daughter's conflicting parentage matters....But when I turn to the mirror in my bedroom to admire us together, I am shocked. She seems so alien. With her long, dark eyelashes and shiny, dark brown hair, she doesn't look anything like me. I know that concentrating on how my daughter looks is shallow. She is a person in her own right, not an accessory to me. But still, I can't shake off the feeling of unease. I didn't realise how much her looking different would matter and, on a rational level, I know it shouldn't. But it does. Evolution demands that we have children to pass on our genes, hence the sense of pride and validation we get when we see our features reappearing in the next generation. With my daughter, I don't have that....But self-flagellation is not useful. I have more pressing concerns. I am now the mother of a 'black' child, even if she is more the hue of weak tea than espresso.... When she was born, pale but with lots of dark hair, I asked the midwife if her eyes would stay blue. 'Asian genes are very strong,' she said in what I took to be an ominous tone. No more Brady Bunch kids for me. The midwife has been proved right and every day my baby's eyes get a little darker."

Since parents share fewer genes with mixed-race children, people involved in interracial marriages are short-changing their own genes, which might explain why people engaged in mixed-race relationships often tend to have lower mate value. A recent survey found that white women who date black men tend to be fatter, dumber and more quarrelsome than average.

Given the very real problems with having mixed-race children (such as the fact that parents often feel estranged from children who do not resemble them and that the child will never fully identify with the ancestral traditions of the parents, esp. the lighter parent), it is unsurprising that mixed-race children suffer more problems of identity and health. For instance, mixed-race people are less likely to survive organ transplants, especially bone marrowtransplants. In general, mixed-race people have more health problems. A study by J. Richard Udry notes:

"A new study that involved surveying 90,000 adolescent U.S. students showed that those who considered themselves to be of mixed race were more likely than others to suffer from depression, substance abuse, sleep problems and various aches and pains. Conducted by researchers at the University of North Carolina at Chapel Hill and the National Institutes of Health, the investigation found that adolescents of mixed race were more likely to have other health problems as well."

In other words, an argument could be made that mixed-race families are maladaptive -- both for the parents and the children -- and undermine one's genetic interests. As noted by various commenters, multiracial families often do not possess the harmony, cooperation and purposefulness of same-race families, because mixed-race families lack the focus of genetic investment and returns that same-race families possess.

Update: Adoption: Regarding transracial adoptions, the same logic would apply. For example, an English family that adopts a black child will be around 109 times more closely related to a random Dane than to their adopted child. As a result, a harmonious congruence of the parents' and adopted child's ethnic genetic interests will be missing. The ethnic genetic interests of the parents and adopted child will often be at odds, creating a disharmonious family structure.